Shutter

The second most important system that controls light reaching your media is your shutter. Unlike the aperture, the shutter is pre-set and does not get into motion until you begin exposure.The shutter consists of two shutter curtains. These curtains are usually put together from 3 narrow overlapping layers of plastic to ease the force put on the whole of the curtain, as it quickly moves from place-to-place during exposure.

During exposure the first shutter curtain is pulled up from covering the media, releasing light into it. When the first curtain reaches the trigger point, when the bottom of the media had enough light reaching it, the 2nd curtain begins to cover it up and moves along as well. If the shutter speed is set to faster than the curtains can handle, curtains do a little cheating trick: the 2nd curtain begins to pull up too before the 1st reaches top.Once the 2nd curtain is all the way up, the mirror of an SLR drops down and in the meantime both curtains (keeping together) drop back down to their original positions.

Blades of the Shutter curtain

Slow shutter speed with panning
Subject is sharp but static things are blurred